The conventional Francis-type runner is composed of a crown section with a concentric band or ring section and an array of blades extending between and interconnecting the crown and hub with the ring. The runner is rotated about the axis of the crown by the action of water passing through a plurality of flow paths formed one between each pair of adjacent blades in the array of blades. Each of the blades has a flow dividing edge which defines the line of demarkation between a suction surface leading in the direction of rotation and a trailing pressure surface. The blades are normally oriented so that the runner slopes from the outlet toward the crown to advance in the direction of rotation of the runner.
One of the problems inherent in conventional runner designs is their susceptability to cavitation damage, particularly at or near the fillet forming the junction of the band and the suction face of the blades immediately downstream of the flow dividing edge in the direction of water flow. Such cavitation has been known to cause severe damage to the blade requiring field repair and in some cases blade modifications which are both very costly and difficult to perform.
Such cavitation also is indicative of poor velocity distribution in the flowing liquid which inherently will reduce the hydraulic efficiency of the equipment. It is obviously advantageous to minimize such hydraulic losses where possible (see for example Canadian Pat. No. 748,229--Sproule et al, dated Dec. 13, 1966.)
Generally the turbine blade is designed or contoured to minimize cavitation and maximize efficiency. In some cases it has been found necessary to surface finish the runner in discrete locations, particularly in the fillet area, forming the junction between the blade and band, for example with a stainless steel overlay. This practice is also expensive and its use generally held to a minimum.